The present invention relates to improvements in a printing press, particularly of the type used in flexographic printing.
Flexography is a specialized form of relief printing which is used on materials having surfaces not easily handled by ordinary letterpress methods, for example for printing plastic packaging. Flexographic presses may also be used for process color printing.
In flexographic presses, an inking roller is supplied with ink from a flexographic ink fountain. The inking roller has a plurality of depressions or cells which are filled with ink, and a reverse angle doctor blade, which bears against the roller surface, scrapes off excess ink from the inking roller surface, so as to leave ink only in the depressions. The inked roller rotates into contact with the printing cylinder, where the ink is transferred to the printing cylinder.
Conventional flexographic presses operate at speeds up to about a thousand feet per minute, which is slower than the operating capabilities of other printing methods, e.g. intaglio printing presses. Flexographic presses are generally incapable of running in speeds in excess of one thousand feet per minute, since two adverse phenomena occur: starvation and foaming. Starvation means that insufficient ink is transferred from the ink fountain to the inking roller to achieve satisfactory printing. Foaming refers to aeration of the ink in the ink fountain, which further exacerbates the problem of starvation.
In addition to the limitation in press speed inherent in flexographic presses, the inks commonly used in flexographic presses, i.e. aniline ink or water based inks, tend to dry quickly. As a result, during a press run ink can begin to dry in the ink cells. A build-up of dried ink in the cells of the inking roller reduces the amount of ink transferred to the printing cylinder, and adversely affects printing quality.